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Glowing sharks sport unusual eyes

Deep in the twilight zone of the ocean, small, glowing sharks have evolved special eye features to maximize the amount of light they see, researchers report this week in PLOS ONE. The scientists mapped the eye shape, structure, and retina cells of five deep-sea bioluminescent sharks, predators that live 200 to 1000 meters deep in the ocean, where light hardly penetrates. The sharks have developed many coping strategies. Their eyes possess a higher density of light-sensing cells known as rods than those of nonbioluminescent sharks, which might enable them to see fast-changing light patterns. Such ability would be particularly useful when the animals emit light to communicate with one another. Some species also have a gap between the lens and the iris to allow extra light in the retina, a feature previously unknown in sharks. In the eyes of lanternsharks, pictured above, the scientists discovered a translucent area in the upper socket. The researchers suspect this feature might help the sharks adjust their glow to match the sunlight for camouflage.